Frame recursive type noise reduction is known as a method of removing noise from a video input signal containing the noise. For example, a known method calculates a difference between a signal obtained by frame-delaying a video output signal (from which the noise is removed) and a video input signal, multiplies the difference value by a desired recursive coefficient (from 0 to less than 1) and conducts addition/subtraction to and from the original video input signal to remove the noise.
Japanese Patent Laid Open No. 2002-94834 and “Transistor Technology SPECIAL” (by Shunya MATSUI, No. 52, 1999, CQ Publication Co., p 89-92) describe other known technologies for motion detection and noise reduction technologies.
An example of the prior art technologies will be hereinafter explained.
FIGS. 18A and 18B show an example of a frame recursive type noise reduction apparatus according to the prior art. Referring to FIG. 18A, a frame memory 1801 frame-delays a video output signal (from which noise is removed) 1807. A subtracter 1802 calculates a frame difference between an output of the frame memory 1801 and a video input signal 1806. This difference value is inputted to a motion detection/recursive coefficient generation circuit 1803 that executes motion detection and generation of a recursive coefficient. The motion detection/recursive coefficient generation circuit 1803 generates the recursive coefficient in accordance with the difference value.
FIG. 18B shows an example of the relation between the difference value and the recursive coefficient.
Referring to FIG. 18B, abscissa 1810 represents a difference value. Ordinate 1811 represents a recursive coefficient. Curve 1812 represents a relation between the difference value and the recursive coefficient. As represented by the curve 1812, the motion detection/recursive coefficient generation circuit 1803 has characteristics such that the recursive coefficient is non-linearly decreased as the difference value becomes great. This represents that motion detection is made by utilizing the correlation that the difference becomes great when the motion becomes great. A multiplier 1804 multiplies the recursive coefficient obtained in this way by the frame difference. An adder/subtracter 1805 further conducts addition/subtraction of the output of the multiplier 1804 to or from the video input signal 1806 in accordance with the sign of the frame difference and a video output signal 1807 from which the noise is removed can thus be obtained.
In the prior art system described above, the noise removal effect becomes great when the recursive coefficient is increased but a problem of tailing that results from the frame difference occurs in this case at a portion having motion. Therefore, a method is known that executes a non-linear processing in accordance with the frame difference value, or detects motion and changes the recursive coefficient in accordance with the detection result. However, since motion detection of the prior art utilizes as such the frame difference value or horizontal and vertical edge detection, the noise and the motion cannot be discriminated for the motion portions having a level substantially equal to or below the noise level. When the noise is removed under this state, the tailing phenomenon occurs. Consequently, under such a state, the noise removal effect is limited from the aspect of prevention of the tailing problem.
Another method executes the non-linear processing in accordance with the difference value, or detects motion and changes the recursive coefficient in accordance with the detection result. This method is described in the non-patent reference given above. However, motion detection according to the prior art utilizes as such the frame or field difference value. In this case, the noise and the motion cannot be discriminated for the motion portion having a level substantially equal to or below the noise level. Under such a state, therefore, the noise removal effect is limited from the aspect of prevention of an after-image.
In the construction described above, when the difference value resulting from the motion in the frame difference is substantially equal to or below the level of the difference value resulting from the noise, they cannot be discriminated and the recursive coefficient cannot be increased to prevent the occurrence of tailing. Therefore, the noise removal effect is limited.
To adaptively improve the noise removal effect for dark portions at which the noise amount is likely to become great, a method that controls the noise removing level by using information from an AGC circuit or the input signal level may be possible. To corporate with the AGC circuit, however, a new control circuit must be added. In the method using the input signal level itself, fluctuation of the control level is likely to occur in a spatial direction due to the influences of noise.
When recursive type noise reduction and non-recursive type noise reduction using a filter in the spatial direction are adaptively used in combination in accordance with the motion detection result, unnaturalness for observation is likely to occur with a discontinuous processing in the spatial direction if the discontinuous motion detection result of the prior art is used. Moreover, because a filter in the spatial direction is applied to a moving picture region containing a contour, obscurity of the contour is likely to occur in the a moving picture and a contour correction circuit must be added.
Generally, a still picture portion and a moving picture portion appear with certain coherence in the image. Therefore, the possibility is high that the same recursive coefficient is used among adjacent pixels in that region from the motion detection result. Particularly when the recursive coefficient is increased to a great value, unnaturalness for observation such as pasting feel of the image is likely to occur. Furthermore, a threshold value for the motion detection becomes great in a low electric field and when the recursive coefficient is increased, unnaturalness such as feeling of a film pasted on the entire screen is likely to occur.